1. Technical Field:
The invention relates to hydrocarbon recovery from a subterranean hydrocarbon-bearing formation and more particularly to a reversible polymer gel used in hydrocarbon recovery applications.
2. Background Information:
Crosslinked polymer gels have widespread utility in hydrocarbon recovery applications. For example, gels have utility as fracturing fluids and workover fluids. Gels also have utility as a permeability reducing material for conformance improvement, zone abandonment, and other fluid diversion processes.
It is often desirable to reverse gels after a specified time period since gels used in many applications have only temporary utility. For example, gels used as fracturing fluids become obsolete once the fractures are formed and preferably are reversed thereafter. Oxidants, such as persulfates, hypochlorites, peroxides, and metal cations are well known reversing agents which reverse polymer gels. Oxidants reverse the gels by degradation, i.e., attacking the polymer at numerous points along its backbone to fragment the polymer molecule.
A second type of gel reversing agent is ligands or chelating agents which form complexes termed chelates with the gel crosslinking agent. These agents reverse the gel by a degelation mechanism rather than a degradation mechanism. Degelation differs from oxidant degradation in that the reversing agent attacks the crosslinks between the polymer molecules rather than the polymer molecule itself.
The chelating agent removes the crosslinking agent from the crosslinking site by complexing with it. Chelating agents initiate degelation immediately upon contact with a crosslinked polymer gel. Thus, the chelating agent must be maintained separate from the gel until degelation is desired. To effect in situ degelation of a subterranean gel, the chelating agent must be withheld from the subterranean formation prior to reversal. Unfortunately, adequate contacting of a mature gel in situ which has penetrated the formation matrix is extremely difficult to accomplish by post-gelation injection of the chelating agent. For this reason, chelating agents presently have limited utility as gel reversing agents.
A process is needed which effectively employs chelating agents for in situ degelation. Specifically a process is needed which effectively utilizes chelating agents for in situ degelation of crosslinked polymer gels penetrating a formation away from a well bore.